The Myanmar (Bamar), Karen, Shan, etc populations were all there before the British drew the borders of 'Burma.' The conflict goes back way farther than WWII. The main difference now is that the Myanmar are winning.
Settlers and natives has nothing to do with it. All the major armed ethnic peoples existed in Myanmar since before WWII. The “settlers” were the British and they left.
I am not convinced this ever happened in any meaningful sense. As you note, it turned into armed conflict almost immediately. This was a British-induced transient state that was unable to sustain itself in the region.
> The “settlers” were the British and they left.
The Myanmar state/people are also colonizers/conquerors. You don't need to come across an ocean to see territory that you dont control and decide to try to take control by force. Facebook and ethno-nationalism are simply new tools in a long-running conflict.